Cold trapping of gaseous chemical samples is known to prior art. A chemical sample is heated to produce a vapor of gaseous sample molecules. The gaseous sample molecules are then combined with an excess of inert gas molecules, known as matrix molecules, and the gaseous mixture is directed onto the surface of a cold substrate. Upon contact with the substrate, the gaseous mixture condenses and freezes, forming a solid deposit on the substrate surface. The solid deposit, called a sample matrix, consists of a matrix formed by the inert molecules intermixed with a random scattering of sample molecules. The sample molecules in the matrix may then be studied using known chemical analysis techniques. In particular, if the substrate and the matrix are made of materials which are transparent to a particular range of electromagnetic radiation, for instance infrared, visible, or ultraviolet radiation, then a transmission absorption spectrum of the sample may be obtained with a suitable spectrometer. Because many important chemical transitions occur in the infrared spectral region, an infrared spectrometer, such as a Fourier transform infrared spectrometer, is usually preferred.
To prevent deleterious effects on the analysis, the sample molecules should be prevented from reacting with each other, or with residual molecules within the vacuum chamber enclosure, to form other compounds. To this end, the relative abundance of the sample molecules, compared to the number of inert matrix molecules, is kept rather low. Usually, the molecules of the sample make up less than one percent of the total molecules in the sample matrix. Because the percentage of sample molecules is so low, each sample molecule in the sample matrix is surrounded by a very large number of inert matrix molecules. The inert matrix molecules form a lattice within which the sample molecules are widely dispersed. This isolates the sample molecules from each other and also from other molecules which may be present in the vacuum chamber, and thereby inhibits the reactivity of the sample molecules. As a result, the technique is well suited to the spectral analysis of unstable or highly reactive molecules, such as free radicals and ionic metals.
To further reduce the reactivity of the sample molecules, the substrate on which they are deposited is cooled to cryogenic temperatures. Cryogenic temperatures, which are defined as being below zero degrees Celsius, reduce the thermal excitation level of the sample and thereby reduce the kinetic energy available for producing chemical reactions. Additional benefits of cryogenic cooling are an increased tendency for the sample matrix to solidify onto the substrate, rather than onto the inner walls of the vacuum chamber enclosure, and an improvement in the spectral quality of the absorption analysis due to a reduction in the peak broadening of the spectral lines.
Within the context of cold trapping, it has become desirable to combine this technique with other gas analysis techniques, particularly those which include gas chromatography, supercritical fluid chromatography or liquid chromatography. For instance, in gas chromatography the sample is passed through a gas chromatograph (GC) prior to being deposited onto the cold trap substrate. The function of the GC is to vaporize the sample and pass the gaseous sample molecules through a separating element. The separating element, usually a packed column or capillary tube, inhibits the flow rate of a molecule by a factor which depends upon the chemical composition of the molecule. As a result, when molecules of a multicomponent sample pass through the GC, they exit as a series of clusters, with each cluster consisting of molecules having essentially the same chemical composition. This concentrates the various components of the sample together and also separates them from each other in time as they leave the GC. A transfer tube, which terminates near the surface of the cold trap substrate, may then be used to carry the clusters of effluent from the GC to the substrate. The clusters of effluent may then be combined with matrix material and deposited in the usual way.
Since the effluent clusters exiting the GC are separated from each other in time and each cluster comprises a concentrated component of the original sample, it would be useful to be able to analyze the spectral profile of each cluster individually. Not only would the resulting spectral identification be simplified, because the sample has been broken down into its constituent components prior to spectral profiling, but the ease of identifying the sample components would be enhanced because the characteristic flow rate information from the GC could be used to augment the spectral information obtained for each cluster. Other devices, which may include the supercritical fluid chromatograph (SFC) or the liquid chromatograph (LC), would offer similar advantages when combined with cold trapping and infrared spectral analysis.
Unfortunately, while some have succeeded in doing this using a spectrometer which performs reflection measurements, no means has been devised to do this commercially using a substrate which is transmissive to infrared radiation. One reason that transmission measurements are preferred over reflection measurements is that the latter may be highly dependent on the morphology of the sample matrix deposit, while the former are not. Another reason is that reflection measurements involve two passes of the radiation through the sample, so sample thickness variations may cause considerable variation in the throughput of the instrument.
The reason that a transmission mode instrument has not been commercially successful is primarily because of a lack of a suitable substrate material. This largely results from the fact that the materials which are transparent to infrared radiation, materials which have traditionally consisted essentially of certain salts such as sodium chloride (NaCl), potassium bromide (KBr), and cesium iodide (CsI), have characteristics which make them extremely poor substrates for cold trapping.
First, these salts have poor mechanical characteristics. They are fragile, and thus easily broken; some are hygroscopic, and thus prone to permanent damage by fingerprints or prolonged exposure to moisture; and some are soft, and thus easily scratched. Optical grade elements are difficult and expensive to produce and may deteriorate very rapidly even after being installed in an artificial environment. Because the salts are soluble in many substances, including water, they can be difficult to work with and may be damaged merely by breathing on them. They may also be damaged if a corrosive effluent sample is deposited onto them. They may also tend to warp out of shape unless they are always kept in a vertical position, because when they are held horizontally they tend to sag in the center. They also tend to sag even if kept in a vertical position because they have a viscosity which allows them to flow very slowly. This physical property also limits the maximum size of the optical grade elements which can be produced. Furthermore, even small optical grade elements are very expensive, both in terms of the initial cost and in terms of the replacement cost which may be required.
Second, in addition to having poor mechanical characteristics, these materials are also extremely poor thermal conductors, which makes them ill suited for use as cold trap substrates. If the substrate is larger than a few square centimeters, the cryogenic cooling apparatus, which is usually a cryogenic refrigerator, may be unable to remove the heat produced by the condensation and solidification of the sample matrix. As a result, the transmissive substrates in current use are usually kept very small and they are mounted directly to the cold tip of the cryogenic refrigerator. A rigid copper frame is often used, together with gaskets made of indium, to increase heat flow from the substrate to the cold tip. Indium gaskets are used because the salt substrates are soluble in many of the thermally conductive pastes ordinarily used to increase heat transfer between connected elements. The copper frame and indium gasket increases the ability of the refrigerator to maintain the substrate at cryogenic temperatures, but causes difficulty if the substrate must be moved to enable cold trapping of more than one sample matrix. Moving the substrate may be important for other reasons. Often, the substrate position must alternate between a deposition position, where the sample matrix is deposited onto the substrate, and an analysis position, where the sample matrix is analyzed with the spectrometer. In addition, if an instrument such as a GC is used to separate the components of the sample prior to deposition, it is desirable to deposit the different effluent clusters on different regions of the substrate surface, and this requires that the substrate be moved relative to the terminal end of the transfer tube. However, if the substrate is rigidly fixed to the cold tip of the cryogenic refrigerator, as is the case with prior art designs, then the entire refrigerator must be moved in order to move the substrate and this requires elaborate and expensive cryogenic bearings which add further cost and complexity to the resulting instrument.
As a result of all of these difficulties, it is currently impractical to produce a general purpose cold trapping instrument which permits transmission absorption spectra to be obtained.